Optimal viewing height of any painting is 5-feet (152cm) from the ground to the center of the painting.
All original paintings are provided ready to hang, with heavy duty saw tooth hangers (screwed into the frame), capable of supporting a minimum of double the weight of the frame and canvas. I have been using these for many years - they are made in Germany and extremely durable.
Paintings that are custom-framed are also ready to hang, with hooks and wires installed by a local custom framing shop used: (http://bilder-hofmann.de/)
For larger paintings (based on overall size and frame thickness), it is best to hang the painting on a hook that is screwed into a wall plug/ anchor (available at any hardware store).
Thin wall nails should never be used for hanging heavy, and especially, valuable paintings! Avoid at all costs!
Viewing angles are important for pictures with a medium to high gloss surface.
Paintings with such surfaces (ideally) should not be hung either perpendicular to, or directly facing windows and bright light sources, as in the example below. This can create viewing 'hot spots' which can obscure parts of the painting.